The following discussion of the prior art is provided to place the invention in an appropriate technical context and enable the advantages of it to be more fully understood. It should be appreciated, however, that any discussion of the prior art throughout the specification should not be considered as an express or implied admission that such prior art is widely known or forms part of common general knowledge in the field.
In a process for the volume manufacture of products such as photovoltaic cells, there is the potential for systematic manufacturing errors to cause defects in the product. For example a piece of foreign material lodged in a print screen can cause a defect such as a crack, scratch or stain in a large number of photovoltaic cells until it is detected and removed. The sooner such problems can be detected and corrected, the less the economic impact.
Irrespective of the type of product being manufactured however, it can be difficult to distinguish process-induced defects from features that are inherent in the product material. In the context of photovoltaic cells manufactured from multicrystalline silicon for example, and as described in Japanese patent application No JP2007-067102 A, it can be difficult for optical imaging techniques (typically involving reflection or transmission of light in the visible or infrared spectral regions) to distinguish process-induced defects such as stains from the grains or grain boundaries of the multicrystalline material.
Photoluminescence (PL) imaging has been proposed as a convenient and fast technique for assessing the quality of semiconductor material being fed into a photovoltaic cell line, and for monitoring the photovoltaic cells throughout their manufacture process and onwards into module assembly, see generally published PCT patent application Nos WO 2007/041758 A1, WO 2007/128060 A1, WO 2009/026661 A1, WO 2009/121133 A1, WO 2011/079353 A1 and WO 2011/079354 A1. After electrical contacts have been printed, luminescence can be generated additionally or alternatively by electrical excitation (electroluminescence, EL). In particular, luminescence (PL and/or EL) from multicrystalline or monocrystalline silicon has been shown to deliver information about many material and electrical properties that can influence the performance of photovoltaic cells, including dislocations, atomic impurities, inclusions, shunts, contact resistance of metal lines, and cracks. Since grain boundaries also influence the electron-hole recombination responsible for the luminescence emission, they can also show up in luminescence images. However in this wealth of information it can be difficult to distinguish one type of feature from another. Image processing algorithms are useful to some extent, working on the principle that different types of features have different dimensional patterns, but these algorithms cannot generally distinguish process-induced defects from intrinsic material features. Similar difficulties apply to other characterisation techniques including optical (reflection or transmission) imaging.